Data signals are degraded when transmitted over long lengths of cable. As the frequency of the data signal increases, the adverse effects on signal quality become more prevalent in the received signal. The data signal will become degraded in amplitude due to loss and phase dispersion from the cable.
Digital data signals consist of an infinite series of frequency components. High frequency digital data signals have very fast edges resulting in larger amplitudes in the high frequency components of the signal. Since the loss in the cable increases with frequency, the higher frequency components in fast edge digital data signals become greatly attenuated over distance when compared to the lower frequency components.
Equalization systems attempt to correct for high frequency losses and signal dispersion over cabling by boosting the higher frequency components of received data signals. High pass filters are used as equalizers to balance the frequency components of the received signal and attempt to reconstruct the fast edges found in the original data signal. The cable can be represented as a transfer function between the original signal and the received signal. By designing the high pass filter to have the inverse transform function of the cable's characteristics, the adverse effects of the cable loss can be eliminated.
The high pass equalization method may be represented as a simple capacitor and resistor network. Since the data signal is effectively AC coupled through the filter, a DC restore circuit is often required to recover a DC baseline for the incoming signal. DC servo circuits are often employed for this purpose. Once a proper DC level has been restored to the equalized signal, the data signal can be detected through a comparator circuit. The comparator and the DC restore circuit may be combined into a single circuit such as through the use of positive feedback from the output of the comparator. The output of such a comparator is a “sliced” version of the received data signal.
Phase locked loop and delay locked loop techniques may be used to provide adaptive equalization. The sliced data signal can be combined in a phase locked loop to adjust the high pass filter such that the transform function of the high pass filter closely matches the inverse transform of the cable.